I don't blame O'Brien for leaving for the NFL since he has stated that being a head coach in the NFL has always been his career goal. I'm disappointed he is leaving after only two years but I also wasn't expecting him to stay for more than 5 years.
Remember that O'Brien agreed to be the head coach before the sanctions were handed down, and while some degree of punishment was expected no one thought that the penalties would be so harsh. O'Brien had the opportunity to leave last season and I believe stayed due to a feeling of loyalty to the players and an increase in pay that was negotiated before beginning of next season.
I am sure that being the new face of PSU was a tough burden to bear for him. With the current university leadership being very unpopular amongst the players and the alumni, BOB had to do way more than others and was getting paid less to do it. (After O'Brien was named the head coach, the entire senior class told him they did not want to see Joyner or Erickson on the field before the games. Joyner must have felt some contrition -- or embarrassment, take your pick -- because he respected their wishes, not setting foot in front of the team again until the 2012 banquet.
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I very much doubt that he left PSU mainly because of the people still loyal to Paterno. I am sure that played a part in his decision, but it is tough for anyone taking over a program after a legend leaves it. Whomever takes over at Texas will have a similar issue as I am sure there are still people there loyal to Mack Brown. Look at Tomlin who still is being compared to Cowher and Noll regarding any decision that is made. If that was the biggest reason, then Houston will be a good place for him as he won't have to accomplish much to be the greatest coach in their history.